Added a very basic XKCD hook

[everydayanchovies]

I created a very basic XKCD hook (https://www.xkcd.com) that downloads the last few comics on launch.

There is one config, num-comics-to-download. If set to 10, the last 10 comics will be fetched if they haven't already been.

I have only been able to verify that it fetches the comics, no tests have been written and the code is otherwise untested. But I figured I'd share it.